Method for shutting down indirect internal reforming solid oxide fuel cell

Abstract

Provided is a method for shutting down an indirect internal reforming SOFC, in which reliable reforming, prevention of anode oxidative degradation, fuel saving and time saving are possible. Reforming catalyst layer temperature T is measured, and FkCALC is calculated; when FkCALC≧FkE, T is measured, and FkCALC and FkMinCALC are calculated; if FkMinCALC≧FkE, then the flow rate of the fuel supplied to the reformer is set to FkE and the method moves on to step D; if FkCALC≦FkMinCALC<FkE, then C6 to C9 are performed in order; C6) the temperature of the reforming catalyst layer is increased; C7) T is measured, and FkCALC and FkMinCALC are calculated; C8) if FkCALC<FkE, then the flow rate of the fuel supplied to the reformer is set to FkMinCALC and the method returns to C6; C9) if FkCALC≧FkE, then the flow rate of the fuel supplied to the reformer is set to FkE and the method moves on to D; D) the method waits for the anode temperature to fall below an oxidative degradation temperature. FkE and the like are defined in the specification.

The invention claimed is: 
     
       1. A shutdown method for shutting down an indirect internal reforming solid oxide fuel cell comprising
 a reformer for reforming a hydrocarbon-based fuel to produce a reformed gas, 
 said reformer including a reforming catalyst layer, 
 a solid oxide fuel cell for generating electric power using the reformed gas, 
 a combustion region for combusting an anode off-gas discharged from the solid oxide fuel cell, and 
 an enclosure for housing the reformer, the solid oxide fuel cell, and the combustion region,
 wherein 
 a flow rate of the hydrocarbon-based fuel supplied to the reformer in a state in which the following conditions i to iv are all satisfied is represented as FkE, 
 
 i) an anode temperature of the solid oxide fuel cell is steady, 
 ii) the anode temperature is less than an oxidative degradation temperature, 
 iii) in the reformer, the hydrocarbon-based fuel is reformed, and a reformed gas having a composition suitable to be supplied to an anode is produced, and 
 iv) an amount of the reformed gas produced is equal to or more than a requisite minimum flow rate FrMin for preventing oxidative degradation of the anode when the anode temperature of the solid oxide fuel cell is a temperature that is equal to or more than the oxidative degradation temperature,
 a flow rate of the hydrocarbon-based fuel supplied to the reformer at a point of time of the start of the shutdown method is represented as Fk0, 
 a calculated value of a flow rate of the hydrocarbon-based fuel capable of being reformed at a measured temperature of the reforming catalyst layer by a reforming method is represented as FkCALC, a type of this reforming method being a type of a reforming method which is performed after the start of the shutdown method, 
 when the anode temperature falls below the oxidative degradation temperature, supply of the hydrocarbon-based fuel to the reformer is stopped to complete the shutdown method, and 
 while the anode temperature does not fall below the oxidative degradation temperature, the shutdown method comprises the following steps: 
 
 A) measuring a reforming catalyst layer temperature T, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE; 
 B) when FkCALC<FkE in step A, performing the following steps B1 to B4 in order:
 B1) increasing a temperature of the reforming catalyst layer, 
 B2) measuring the reforming catalyst layer temperature T, calculating FkCALC using this measured temperature T, and comparing values of this FkCALC and FkE, 
 B3) when FkCALC<FkE in step B2, returning to step B1, and 
 B4) when FkCALC≧FkE in step B2, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer from Fk0 to FkE and moving on to step D; 
 
 C) when FkCALC≧FkE in step A, performing the following steps C1 to C5 in order:
 C1) measuring the reforming catalyst layer temperature T, calculating FkCALC and FkMinCALC using this measured temperature T, said FkMinCALC being a flow rate of the hydrocarbon-based fuel at which the reformed gas at the flow rate FrMin can be produced in the reformer, and comparing values of this FkMinCALC and FkE, 
 C2) when FkMinCALC≧FkE in step C1, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkE and moving on to step D, 
 C3) when FkMinCALC<FkE in step C1, comparing values of FkMinCALC and FkCALC which have been calculated in step C1, 
 C4) when FkCALC>FkMinCALC in step C3, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkMinCALC and returning to step C1, and 
 C5) when FkCALC≧FkMinCALC in step C3, performing the following steps C6 to C9 in order: 
 C6) increasing the temperature of the reforming catalyst layer, 
 C7) measuring the reforming catalyst layer temperature T, calculating FkCALC and FkMinCALC using this measured temperature T, and comparing values of this FkCALC and FkE, 
 C8) when FkCALC<FkE in step C7, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkMinCALC and returning to step C6, and 
 C9) when FkCALC≧FkE in step C7, adjusting the flow rate of the hydrocarbon-based fuel supplied to the reformer to FkE and moving on to step D; and 
 
 D) waiting for the anode temperature to fall below the oxidative degradation temperature. 
 
     
     
       2. The method according to  claim 1 , wherein the hydrocarbon-based fuel comprises a hydrocarbon-based fuel having a carbon number of two or more. 
     
     
       3. The method according to  claim 2 , wherein a concentration of a compound having a carbon number of two or more in the reformed gas is 50 ppb or less on a mass basis.